Preheat the oven to 350 F.
Peel and half the sweet potatoes.
Bring a pot of water to a full boil and cook the potatoes for about 20 minutes. Drain and set aside.
Rinse and pat dry the chicken breasts.
Slice them in half at an angle to create 4 smaller pieces.
Mix the garlic, onion, chopped thyme, <g class="gr_ gr_217 gr-alert gr_gramm Punctuation only-ins replaceWithoutSep" id="217" data-gr-id="217">salt</g> and pepper together and rub all over the chicken pieces.
In a large saute pan melt the butter and add the shallot. Cook until soft, about 2-3 minutes.
Add the Pancetta and cook for a further 5 minutes or until the Pancetta is cooked and turning crispy.
Add the wine and deglaze the pan scraping the bottom of the pan with a wooden spoon.
Cook for about 3-4 minutes and then add the chicken.
Sear the chicken for 2 minutes on each side and transfer everything from the pan to an oven-proof dish. I used a 10-inch casserole dish.
In the same pan, without cleaning it melt the butter (2 tbsp)
Add the flour and whisk until it starts to turn light golden in color.
Combine the milk and stock and gradually pour small amounts into the pan.
It will thicken up but whisk well after each addition and you should have no lumps.
Continue until all of the liquid has been added and simmer for about 3-4 minutes and the sauce starts to thicken.
Season with some salt and pepper.
Roughly chop the sweet potatoes into 1-inch chunks and arrange in the same dish as the chicken.
Pour the sauce all over the chicken and sweet potatoes.
Bake in the oven for about 45 minutes uncovered until bubbling.
Test a sweet potato by piercing it with a sharp knife. It should go into it easily. If not cook for a further 5 minutes and test again.
Switch your broiler to high.
Sprinkle the cheese over the casserole and place under the heat.
Cook until all of the cheese has melted and is bubbling and starting to brown.
Sprinkle with some more fresh thyme and serve at once on <g class="gr_ gr_114 gr-alert gr_spell ContextualSpelling" id="114" data-gr-id="114">it's</g> own or over brown rice.